The availability of quantitative information representative of the coefficient of friction of the road surface is very beneficial to drivers of moving vehicles, including planes. An exact knowledge of how slippery the surface is, continuously provided, gives the driver significant advantages in determining safe speeds, distances from other automobiles, acceleration and braking patterns etc. so as to permit a safer operation of the vehicle, plane or apparatus being driven.
Previous devices for monitoring the coefficient of friction had significant disadvantages such as, for example: a failure to provide continuous road friction data over long distances; the use of complex electro-mechanical-hydraulic mechanisms with unfavorable wear characteristics; limited, specificity, variability and responsiveness of test wheel(s) braking force; the use of only indirectly measured values of the force (torque) required to produce slippage; employment of equipment which was unsuited for continuous use close to the ground and in inclement conditions; the use of a cumbersome test wheel suspension and carriage; employment of equipment which produced adverse affect on normal drive operation; and the use of relatively complex designs which produces an expensive apparatus of only limited use.
When measuring the coefficient of friction, various means of applying vertical force have been employed, such as: springs, coils, rubber, compressed air bags, pistons containing air, gases, oil. The foregoing vertical force applying devices have significant limitations. The vertical force tends to increase as tensioning means are compressed and decrease when decompressed. The vertical force is difficult to measure, especially if continuous or frequent measurements are desirable. Further, one is generally not able to standardize the force from one system to another. The vertical force tends to change with use/wear due to deterioration of compressed materials/mechanisms, and to vary with change in temperature and/or moisture.